Files I've emailed from calibre have started showing up in my Personal Document Library thing, though I can't archive/retrieve them yet presumably as my Kindle is still on 3.2.

From the help page it appears that they'll also do syncing of position/notes etc. through whispersync, I don't think this used to happen did it?

The page is also clear this won't work on free Kindle apps or the cloud reader. I personally doubt they'll add this any time soon, as it seems like it'll add all the great convenience features to books not sourced through Amazon.